1. Field of the Invention
The present invention relates to a demodulator suitable for use in, for example, a direct-sequence spread spectrum communication system employing offset quadrature phase-shift keying (OQPSK) modulation.
2. Description of the Related Art
Conventional OQPSK demodulators have been described in various publications of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan, including IEICE SB-3-5 (1988, p. 1-564), IEICE B-150 (1991, p. 2-150), IEICE B-200 (1992, p. 2-200), and IEICE SAT 92-2 (1992, pp. 3-8). These demodulators generally employ synchronous detectors that compare the phase of the received OQPSK modulated signal with the phase of a synchronized reference carrier signal. Various synchronous detection methods are employed, but all require at least a carrier recovery circuit to generate the reference carrier signal from the received OQPSK signal, a clock recovery circuit, a bandpass filter (BPF), and a low-pass filter (LPF). These circuits take up considerable space, especially the carrier recovery circuit, which typically includes a phase-locked loop or a reverse modulator.
An OQPSK demodulator employing synchronous detection is unavoidably large and complex. There is a need for a smaller and simpler type of OQPSK demodulator.